“Battery capacity” is a measure (typically in ampere-hours) of the charge stored by the battery, and is determined by the mass of active material contained in the battery. The battery capacity represents the maximum amount of energy that can be extracted from the battery under certain specified conditions.
The effective storage capacity of the battery, however, diminishes with age and undergoes irreversible damage. This damage is caused by various mechanisms including corrosion and other chemical processes, and aging of the internal battery components contributes to the damage as well. As individual plates of the cells corrode, their operational surface area diminishes and the electrolyte undergoes chemical changes, causing them both to be less chemically reactive. These changes reduce the volume of reactive components in the cell, reducing the charge capacity of the cell while also increasing the internal resistance of the cell, as the corrosion products inhibit the free flow of electrons through the plates. Each charge/discharge cycle of the battery also has a similar effect but at an accelerated rate. The end result is that as the battery ages and deteriorates, the effective capacity of the battery decreases, reducing the amount of time the battery can supply power to a device.
One indicator of the battery's ability to retain charge and ability to power a device is the battery “state of health.” Many applications use this parameter to estimate battery performance, for example the “run-time” of the battery, which reflects the amount of time the battery will continue to provide power before it dies. An accurate estimate of the run-time is desired to provide alerts to a user.